As a method for forming a material film of a transistor, there is a sputtering method, which is a deposition method used when a material having low vapor pressure is deposited or when film thickness needs to be controlled precisely. A sputtering method uses a deposition apparatus which can be operated very easily, and thus is industrially used widely.
A sputtering method is a deposition method using a sputtering target, and a sintered body, or a single crystal in some cases, of metal, metal oxide, metal nitride, metal carbide, or the like is used as the sputtering target.
In a sputtering method, after the pressure in a deposition chamber of a deposition apparatus is reduced by a vacuum apparatus, a rare gas such as argon is introduced into the deposition chamber, and glow discharge is caused between a deposition substrate and a sputtering target to generate plasma, using the deposition substrate as an anode and the sputtering target as a cathode. Thus, positive ions in the plasma are made to collide with the sputtering target, and particles which are components of the sputtering target are sputtered to be deposited over the deposition substrate, so that a material film is formed.
A sputtering method is classified depending on a method used for generating plasma, and a method using high-frequency plasma is referred to as a high-frequency sputtering method (also referred to as an RF sputtering method), and a method using direct-current plasma is referred to as a direct-current sputtering method (also referred to as a DC sputtering method).
Since a less expensive power supply facility is used and deposition rate is higher in the DC sputtering method than in the high-frequency sputtering method, the DC sputtering method is more industrially used than the high-frequency sputtering method, in consideration of productivity and manufacturing cost.
In general, a film of a metal material used for a wiring or the like is often formed by the DC sputtering method, and an insulating film is often formed by a PCVD method or the high-frequency sputtering method. In the high-frequency sputtering method, deposition can be performed even when a sputtering target having an insulating property is used. Further, a technique in which a transistor including an oxide semiconductor is manufactured and applied to an electronic device or an optical device has attracted attention. For example, a technique of manufacturing a transistor by using zinc oxide or In— Ga—Zn—O-based oxide as an oxide semiconductor, and using the transistor for a switching element of a pixel of a display device and the like is disclosed in Patent Document 1 and Patent Document 2.